I have been reading the docs, looking for a way to acheive referential
integrity as it is done in RDBMS.

For instance, if I want to constraint the value of atribute title to a
set of value, so that any modification to a value not in the list would
be denied. Let's say the set of legal values could be stored in the
directory, ad objects of a dedicated class;

How can it be done? slapo-refint is not about denying changes,
slapo-constraint can only enforce regex matching.. Is there anything
else I missed?